1. Field of the Invention
The present invention relates to a viewing tube for a microscope and more particularly to a viewing tube for a microscope whose tilt angle can be changed continuously.
2. Related Background Art
Conventionally, in a viewing tube for a microscope, an eyepiece portion can be inclined in accordance with an observer's physique and posture prior to observation. For that purpose, an optical deflecting device needs to be inclined so as to make its deflection angle coincide with the tilt angle of the eyepiece portion. A conventional viewing tube for a microscope will be described with reference to FIGS. 5 and 6. FIG. 5 is a cross section of the viewing tube and FIG. 6 is a cross section showing a principal portion thereof.
In FIG. 5, an eyepiece portion 41 is rotatable around a shaft 42. A gear 43 is provided coaxially with the shaft 42 and fixed to the eyepiece portion 41. The shaft 42 is fixed to a case 44, to which a prism 45 is mounted. The case 44 is rotatable around a shaft 47 fixed to a base 46. A gear 48 is fixed to the base 46 so as to be coaxial with the shaft 47. The gear 43 is meshed with the gear 48. A lower portion of the base 46 is provided with a ring mounting 46a whose cross section is dovetail-shaped. The viewing tube is connected to a main body of the microscope via the ring mounting 46a.
An optical axis of incident light from an objective (not shown) provided on the main body of the microscope is deflected by the prism 45 and enters eyepiece 50 via the eyepiece portion 41 along an optical axis 50a of the eyepiece portion 41.
Referring to FIG. 6, when the eyepiece portion 41 is rotated around the shaft 42, the case 44 rotates around the shaft 42. If the number of teeth of the gear 43, the angle of rotation thereof, the number of teeth of the gear 48, the angle of rotation of the case 44 around the shaft 47 are Za, .theta.a, Zb and .theta.c respectively and Za=Zb, .theta.a=2.theta.c. Therefore, when the eyepiece portion 41 is rotated around the shaft 42 through the angle of .theta.a to change its tilt angle, the prism 45 is rotated through the angle of .theta.a/2, whereby the optical axis 49 of the incident light is deflected at the angle of .theta.a to coincide with the optical axis 50b of the eyepiece portion 41.
The above-described arrangement suffers from a number of disadvantages. For one thing, the gears are expensive. Further, in order to eliminate the backlash between the gears due to the error of the distance between the centers of the gears, the error of the pitch diameter of each gear or the like, it is necessary to adjust the distance between the centers of the gears or to mesh the gears with each other by displacing one of the gears in the circumferential direction. Therefore, the structure or the adjustment in assembly becomes complicated.